1. Field of the Invention
The present invention relates to magnetic toner for two-component developer consisting of magnetic toner containing magnetic particle in a binder resin and toner carrier, and image forming method of visualizing a latent image on a latent image retaining member using the two-component developer
2. Description of the Related Art
In an image forming apparatus such as a laser printer utilizing electrophotographic method, electrostatic copy-machine, plain paper facsimile, and complex machine, first the surface of a latent image retaining member is charged evenly by charging means, then an electrostatic image is formed thereon through exposing by exposing means such as a semiconductor laser and light emitting diode, and then the electrostatic image is visualized as a toner image through developing or inverse-developing by developing means. Then the toner image is transferred directly onto the surface of a recording medium such as a sheet paper, or transferred onto the surface of an intermediate transfer member and then onto the surface of a recording medium such as a sheet paper. The transferred image is then fused in place by fusing means to end the image forming method.
Method of developing electrostatic images into toner images can be broadly classified into two categories, i.e. a dry-type and wet-type, of which the dry-type developing method is in widespread use in these days. The dry-type developing method can be assorted into two kinds by the kind of toner used, developing methods using magnetic toner which is composed of binder resin and toner to which magnetic particle is added internally or externally (single-component magnetic developing, two-component magnetic developing, etc.) and developing methods using toner without addition of magnetic particle (single-component nonmagnetic developing, two-component nonmagnetic developing, etc.).
In two-component developing methods, fine shapes of toner are retained on the surface of the carrier particle of iron or ferrite particle that is relatively large compared to the toner particle, by electrostatic force generated by the friction of the toner shape and carrier shape when they are mixed and stirred. When the mixture is transferred toward a photoreceptor in the form of a magnetic brush and comes adjacent to the electrostatic latent image, the attraction exerting to the toner particles to pull them toward the electrostatic latent image due to the electric field effected by the latent image surmounts the bonding force between the toner and carrier, and the toner adheres on the surface of the electrostatic latent image, thus the electrostatic latent image is visualized.
Although mainly nonmagnetic toner not containing magnetic power is used in a two-component developing method, to use magnetic toner for two-component developer has long been known. For example, in patent literature 1 (JP56-106249 A (1981)) and patent literature 2 (JP 59-162563 A (1983)) is recited to use toner containing magnetic particle. By allowing the toner for a two-component developer to contain magnetic particle, problems are often experienced in two-component developing that scattering or dusting of toner, i.e. flying of toner in all directions, or peeling or falling off of toner from the magnetic roll can be prevented and background fogging can be suppressed. Further, overcharging due to repetition of mixing and stirring of toner and carrier can be prevented by the leak of charge through the magnetic particle.
On the other hand, in recent years there is a demand for high speed, small size, and long lasting image forming apparatuses.
For a high speed apparatuses for mainly business use, it is demanded to quicken the initial rise of charge amount of toner and to retain the charge amount stable compared to conventional apparatuses in order to prevent decrease in resolution and degradation in image quality accompanying increased printing speed.
For a middle speed apparatuses intended for use in small offices and in homes, quick initial increase of charge of magnetic toner is necessary in order to shorten warm-up time after turning on power as far as possible, for turning-on and cutting-off of power are repeated frequently.
Further improvement in resolution, image quality, durability of magnetic toner, and stability under various environments are continuously demanded for image forming apparatuses irrespective of image forming speed depending on use.
Therefore, it is required for magnetic toner that charge amount increase quickly, proper charge amount is resulted always and maintained for a prolonged time without inducing insufficient charge amount and overcharging of toner under high temperature and high humidity environment under which electric charging of toner is difficult and under low temperature and low humidity environment under which toner tends to be electric charged excessively, and as a result a good image characteristic (high density of images without background fogging and good image quality) can be maintained stably for a prolonged time period under various temperature and humidity environments.
Scattering or dusting of toner, toner spent phenomenon in which the toner component adheres precipitates like a film on the surface of the carrier, and developing of scratched streaks on the photoreceptor drum by the magnetic brush, which have been problems when using a two-component developer, tend to occur more easily as printing speed and machine life are increased, so it is also required to evade these problems. However, these demands are becoming difficult to be sufficed enough in the current trend of increased printing speed and smaller size of image forming apparatuses.
Among the problems, development of scratched streaks on the photoreceptor drum which is a latent image retaining member by the magnetic brush is caused by the long time contact of the magnetic brush with the photoconductive surface of the photoreceptor. In a two-component developing method, a magnetic brush composed of two-component developer is formed on the magnetic roll on the part facing the photoreceptor, and image developing is carried out by bringing the magnetic brush into contact with the photoreceptor drum, so the photoreceptor drum is contacted with the magnetic brush for more and more prolonged time period as machine life is extended, and as a result the development of scratched streaks occurs. Heretofore, it has been tried to select carrier to cope with this problem.
However, in the case magnetic particle is contained in a two-component developer, there is possibility of developing the streaks by the magnetic brush by the influence of the magnetic particle.
The spent is a phenomenon that the toner component adheres on the surface of the carrier, and primary function of the carrier is lost when the surface of the carrier is covered with the toner component. Therefore, this is thought as deterioration of the carrier. When the spent phenomenon occurs, decrease in image density occurs and when things come to the worst the developer itself must be replaced by a new one. This spent has been a problem accompanying increased machine life, and possibility of its occurrence is increased in the situation that low temperature fusing of toner has prevailed in recent years. Further, when magnetic toner is used, there exists magnetic constraining force in addition to electrostatic force existing between the toner and carrier, so possibility of occurrence of the scratched streaks increases.
At present time, as a magnetic particle contained in toner, is generally used a particle of spherical shape (see FIG. 1(a)), or polyhedral shape such as hexahedral shape such as cube having six square faces or cuboid having six rectangular faces (see FIG. 1(b)) or octahedral shape having eight triangular faces (see FIG. 1(c)). However, in the case of a magnetic toner containing magnetic particle of polyhedral shape, electric charge tends to be discharged from pointed vertexes and sharp edges of the magnetic particle exposed on the surface of the toner, i.e. the binder resin, and leak of electric charge tends to occur more than necessary. Further, as a magnetic particle of polyhedral shape is low in fluidity and as a result dispersibility in binder resin deteriorates, so it is difficult to allow the magnetic particle to disperse evenly in binder resin. Therefore, variations tend to occur in the dispersion of magnetic particle particles in each of toner particles and easiness to be electric charged and charge amount of each of toner particles tend to scatter.
Accordingly, there is a problem that, with magnetic toner containing magnetic particle of polyhedral shape, quick rise of electric charge amount is difficult, and in addition charge amount itself is small, as a result scattering or dusting of toner and peeling or falling off of toner from the magnetic roll tend to occur, thereby inducing occurrence of flaws in images such as background fogging. Further, as easiness to be charged and charge amount vary depending on the temperature and humidity when the image is formed, there is a problem that the tendency of incurring flaws in the image under an environment of high temperature and high humidity, under which electric charging is difficult, becomes stronger.
Further, when magnetic toner containing magnetic particle of polyhedral shape is used, there is a problem that the pointed vertexes and sharp edges of the magnetic particle exposed on the toner cause magnetic brush streaking which does not occur easily in the case of toner not containing magnetic particle. But, on the other hand, there is an advantage that the pointed vertexes and sharp edges of the magnetic particle have an effect to prevent effectively the occurrence of spent phenomenon of toner to carriers due to their grinding effect.
On the other hand, in the case of magnetic toner containing magnetic particle of spherical shape which has no pointed vertex and sharp edge, the surface of the photoreceptor is hardly scratched by the magnetic particle exposed on the surface of the toner in the magnetic brush formed of the developer. This magnetic particle of spherical shape is better in fluidity, good in dispersibility in the binder resin compared to that containing magnetic particle of polyhedral shape, and can be easily dispersed evenly in the binder resin, so occurrence of variations in dispersion of magnetic particle particles in each of the toner particles is prevented, and occurrence of uneven distribution of magnetic particle in each magnetic toner particle is prevented and thereby easiness to be charged and charge amount of each of toner particles become uniform.
However, as the magnetic toner containing magnetic particle of spherical shape has no pointed vertex and sharp edge, occurrence of spent phenomenon can not be prevented and further release of charge from the magnetic particle exposed on the surface of the toner is hard to be expected, and there is a tendency of overcharging of toner. Therefore, when mixing and stirring of toner and carriers is repeated, toner tends to be excessively charged, that is, overcharge of toner tends to occur. Once the overcharge occurs, image flaws such as decrease in image density tend to occur.
Magnetic particle of various shapes has been investigated in order to take advantage of magnetic particle of spherical shape and of polyhedral shape. For example, magnetic particle of hexahedral or octahedral shape with its vertexes and edges being chamfered to form faces smaller than the faces of the hexahedron or octahedron is disclosed in JP11-153882 A (1999), JP2000-162817 A, and JP2000-242029 A (patent literature 3, 4, and 5 respectively). However, there still exist sharp edge lines formed by the faces of the polyhedron and the chamfered faces, and electric charge tends to be released form these edge lines. Therefore, there is still a possibility that image flaws such as decrease in image density and occurrence of background fogging due to charge leak from the magnetic particle, and when used as two-component developer, the surface of the photoreceptor is scratched by the magnetic brush.
In JP3584954 B (patent literature 6, JP9-59024 A (1997)) is disclosed a magnetic particle of cubic shape with each edge thereof is rounded to form a curved surface. However, in this magnetic particle, vertexes are also rounded together with the edges and there remain no sharp vertexes and edges, so release of electric charge tends to be difficult like spherical shape, and there is a possibility that the magnetic toner is overcharged and image flaws such as decrease in image density occurs particularly under low temperature and low humidity environment. Furthermore, when used as a two-component developer, occurrence of spent phenomenon can not be prevented enough due to the shape of the magnetic particle which is a curved surface on the whole.
In aforementioned patent literatures 1 and 2, suppression of scattering or dusting of toner and occurrence of background fogging is cited as advantage of two-component magnetic toner, however, behavior when forming a large number of images is not recited let alone the influence of the magnetic particle shape, and there is a possibility that stable charge amount can no be secured after forming a large number of images and streaks by the magnetic brush and spent phenomenon occur.
Of recent years, advancement of copying machines and printers, which adopt electrography using two-component developer, in speeding up of printing, downsizing of machine, and increasing of durability and operating life of machine is remarkable, and high resolution, superior image quality, and increased durability are required as a matter of course. Therefore, in order to attain an image characteristic and durability in accordance with increased printing speed, toner of a stable charging characteristic and superior performance is absolutely necessary.